The present invention relates generally to x-ray tubes and, more particularly, to mitigating a gas load on an x-ray tube chamber having a shaft of an anode passing through a ferrofluid seal.
X-ray systems typically include an x-ray tube, a detector, and a bearing assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner or an inspection CT device.
X-ray tubes include a rotating anode structure for distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearing assembly, which typically includes tool steel ball bearings and tool steel raceways positioned within the vacuum region, thereby requiring lubrication by a solid lubricant such as silver. In addition, the rotor, as well, is placed in the vacuum region of the x-ray tube. Wear of the lubrication and loss thereof from the bearing contact region increases acoustic noise and slows the rotor during operation. Placement of the bearing assembly in the vacuum region prevents lubricating with wet bearing lubricants, such as grease or oil, and performing maintenance on the bearing assembly to replace the solid lubricant.
In addition, the operating conditions of newer generation x-ray tubes have become increasingly aggressive in terms of stresses because of g forces imposed by higher gantry revolution speeds and higher anode rotational speeds. As a result, there is greater emphasis in finding bearing solutions for improved performance under the more stringent operating conditions. Placing the bearing assembly and rotor outside the vacuum region of the x-ray tube by use of a hermetic rotating seal, such as a ferrofluid seal, allows the use of conventional lubricants, such as grease or oil, to lubricate the bearing assembly. In addition, maintenance may be performed on the bearing assembly and rotor without interrupting the vacuum in the vacuum region.
A ferrofluid seal typically includes a series of annular regions between a rotating component and a non-rotating component. The annular regions are occupied by a ferrofluid that is typically a hydrocarbon-based or fluorocarbon-based oil with a suspension of magnetic particles therein. The particles are coated with a stabilizing agent, or surfactant, which prevents agglomeration of the particles in the presence of a magnetic field. When in the presence of a magnetic field, the ferrofluid is caused to form a seal within each of the annular regions. The seal within each annular region, or stage, can separately withstand pressure of typically 1-3 psi and, when each stage is placed in series, the overall assembly can withstand pressure varying from atmospheric pressure on one side to high vacuum on the other side.
The ferrofluid seal allows rotation of a shaft therein designed to deliver mechanical power from the rotor on one side of the seal to the anode on the other side. As such, the motor rotor may be placed outside the vacuum region to enable conventional grease-lubricated or oil-lubricated bearings to be placed as an anode support on the same side of the seal as the rotor. Furthermore, such bearings may be larger than those typically used on the vacuum side.
While ferrofluid seals hermetically seal one side from the other, gas and water vapor may diffuse through the ferrofluid and into the vacuum region, the rates of which are governed by diffusion mass transport. Ionizable gases that transport through the seal, when exposed to the high voltage environment of an x-ray tube, can lead to ionization failure of the x-ray tube. Likewise, a gas evaporated from the hydrocarbon-based or fluorocarbon-based oil used to make the ferrofluid that occupies the annular regions of the ferrofluid assembly may pass into the vacuum region, leading eventually to ionization failure of the x-ray tube.
Therefore, it would be desirable to design an apparatus to mitigate the ionizable gas load which either passes through a ferrofluid seal or is evaporated therefrom.